Telegraph cable with loading coils



Mai-ch 20, 1928. 3 1,663,353

, H. SALINGER ET AL TELEGRAPH CABLE WITH LOADING COILS Filed Feb: a. 192

Patented Mar. 20, 1928.

1 UNITEDISTATEST HANS SALINGER Ann zronnnn s'rAHL, or BERLIN, GERMANY.

TELEGRAPH GABLE WITH LOADING- COILS.

Application filed February 8, 192e, Serial in. 86,956, and in Germany February in, 1925.

It is known that the inductance of a cable assists the propagation of telegraph signals along the cable. As in the case of telephony the artificial increase in the inductance of the 5 cable may be effected by surrounding the copper conductor with a material of high ermeability or byinserting coils at regular intervals. In telephonythe length. of these intervals is determined by thev Pupin rule.

According to the latter the natural frequency of a coil section, viz,,portion of the cable between two consecutive coils mustlie above the highest frequency which can still be transmitted over the cable. The important factor in direct current telegraphy is a different one. When direct current telegraph signals passthrough 'acoil loaded cable such as shown in the accompanying drawing, in which 5 are the loading coils and a the distance between the coils they are subject, in. addition; to the distortion which exists in every cable, to an additional change in their shape whicharises from the crowcL ing of the inductance at the points where the coils are. placed. As in the case of telephony, also in this case it is important that the coils shall not be arranged 'too far apart from one'another, as otherewise the ad ditional distortion referred to will become too great. Now, as in the case of telephone cables also in the case of direct current'telegraph cables the required condition may be expressed in a formula containing a relation between two frequencies; For this purpose use is made'of the known conception of telegraphing or transmitting frequency, that is to say, of that fundamental frequency which corresponds to a series of uniform changes (spacing and signalling current) which are transmitted with the desired telegraphing or transmitting speed. This frequency isgenerally referred to in the literature as dot frequency. 7 H

Both experiments and calculation show that the distance between the coils must be so chosen that the natural frequency of'a coil section is at least three times the direct cur rent telegraphing frequency. This rule which is entirely analogous to the Pupin rule determines the admissible maximum distance between two coils of a telegraph cable. The natural frequency of a coil section is fi m 'IT'JECE and thus we obtain for the natural fre-' *T f as 5 PATENT OFFICE.

a C and L are given, so that the above forinulaactually determines the distanccs hetween the coils.

Tlienatural frequency of a (301i section inay be increased beyond three times the telegraphing frequency. However, in that case, for the same total. inductance, usehas to be made of a greater number of coils, which renders the cable more expensive. The reason why an increased natural frequency may sometimes be advantageous is that the more numerous coils are smaller and can be laid more easily. I Thus for instance,: if the natural frequency is made sixtimes instead of three times the telegraphing frequency, assuming the distance between the coils to be reduced to one-half, the inductance of the coils can be reduced to one-half and the coils thus have a smaller weight, whereby their laying. is correspondingly facilitated. 

